1. Field of the Invention
The present invention relates to an image input apparatus for inputting image data to an image processing apparatus by using an image sensor having a wide dynamic range (e.g., 60 dB or more).
2. Description of the Related Art
In general, image sensing devices have a dynamic range with a linear input-output characteristic, so the dynamic range is limited, at maximum, to two digits (i.e., several tens dB) or so. Accordingly, there has been a problem that when the light and dark difference (dynamic range) of a subject is large (e.g., 60 dB or more), the output of an image input apparatus is saturated with respect to light in a high luminance region, so that a white blown-out (white degradation) or black block out (black degradation) phenomenon occurs, thus making it impossible to take a picture of an object that actually exists there.
In contrast to this, there has been proposed an image input apparatus which is able to take a picture of a subject even with a dynamic range of five digits or more without its output being saturated, by using an image sensing device with an output characteristic other than a linear characteristic (e.g., logarithmic compression) (see, for instance, a first patent document (Japanese patent application laid-open No. H3-192764)).
In addition, there also has been proposed an image sensing apparatus which is improved in contrast in a low luminance region by combining both of a linear characteristic and a logarithmic characteristic, and controlling a change point between the linear characteristic and the logarithmic characteristic in accordance with the brightness range of a subject to be sensed or imaged see, for instance, a second patent document (Japanese patent application laid-open No. 2004-88312)).
In the known image input apparatuses, however, there are the following problems. That is, for example, in case of the above-mentioned first patent document, it is difficult to provide an output stage with a dynamic range similar to that of the amount of incident light, and hence contrast is substantially reduced, thus making it hard to obtain a sufficient contrast as required for image processing such as character recognition, object detection, etc. Also, in case of the above-mentioned second patent document, a contrast as intended by an image sensing side or photographer cannot necessarily be obtained as in the case where a subject to be sensed is located in a high luminance region, for instance.
Moreover, controlling the characteristic of an image sensing device in accordance with the brightness range of a subject to be sensed, as in the image sensing device disclosed in the above-mentioned second patent document, is synonymous with changing the dynamic range of the image sensing device into a linear characteristic or a logarithmic characteristic so as to match the dynamic range of the subject. Accordingly, there arises the following problems. That is, when the dynamic range of the background is changed for instance, it is convenient in image processing to take a picture of the subject while keeping the contrast of the subject rather than moving the dynamic range of the image sensing device, but on the other hand, when the dynamic range of the image sensing device is changed in accordance with the brightness range of the subject, an image suitable for an image processing apparatus can not necessarily be obtained. In particular, for an image processing apparatus requiring recognition processing, reduced contrast of an object to be recognized directly means reduction in the recognition rate, and can be a main cause for impairing merchantability or marketability.